Project 1 Summary/Abstract Project 1 of the MSKCC-UW/Fred Hutch Prostate Cancer Drug Resistance and Sensitivity Center focuses on acquired resistance to hormone therapy that is caused by androgen receptor (AR) pathway reactivation. The project consists of two Aims, based on two different mechanisms of AR pathway activation. Aim 1 focuses on the glucocorticoid receptor, which we and others have shown can substitute for AR to drive tumor growth through a bypass mechanism. Clinical datasets, albeit limited at this stage, suggest that this mechanism could be responsible for 20-30% of acquired resistance to enzalutamide. The experiments in Aim 1 will explore the preclinical activity of treatment regimens that target AR in combination with a competitive antagonist of GR (from ORIC Pharmaceuticals) or with a BET domain inhibitor (from Constellation Pharmaceuticals) that potently downregulates GR expression. Aim 2 addresses the more common circumstance where alteration of AR itself is responsible for AR pathway reactivation, a circumstance present in more than 50% of acquired resistance patients. This can occur through AR gene amplification, AR mRNA splice variants or AR mutations, roughly in that order of frequency. Experiments in Aim 2 are designed to inhibit AR more potently than is currently possible with current antagonists such as enzalutamide, first by interfering with the transcription of AR and its downstream target genes using a CBP/p300 inhibitor (from AbbVie). We will also evaluate a tool compound targeting the chromatin modifier G9A/EHMT2 that we recovered in a whole genome shRNA screen, based on selective antiproliferative activity in combination with enzalutamide. Finally, we will conduct a screen for drug targets that further impair AR transcriptional output beyond that seen with enzalutamide. In total, we will evaluate four AR combination therapy regimens, three of which involve drugs that are currently in clinical development or soon will be. In summary, this Project has the potential to generate multiple mechanism-based combination therapy clinical trials together with insights into how to select patients using clinically feasible biomarkers. Furthermore, the findings could be more broadly relevant to other hormone receptor dependent diseases such as breast cancer.